1. Field of the Invention
The present invention relates to a reticle and an exposure apparatus using the reticle, and more particularly, to the use of such items with particularized alignment marks formed thereon to detect and adjust the alignment state thereof.
2. Description of the Prior Art
Generally, an exposure apparatus is used in a photolithography process for manufacturing a semiconductor device. The exposure apparatus functions to form a circuit pattern on a wafer by projecting the circuit pattern onto a target region of a wafer coated with a photosensitive material (photoresist) layer through a reticle (also referred to as a “mask”), thereby imaging the pattern of the reticle on the wafer.
Conventional semiconductor manufacturing forms a plurality of semiconductor chips next to one another on a wafer, with each chip typically corresponding to a particular target region. The pattern of the reticle is repeatedly imaged on each target region one by one.
Generally, the exposure apparatus can be divided into stepper-type apparatus and scanner-type apparatus. The stepper functions to form an entire pattern on a target region by exposing the target region once, while the scanner (or step-and-scan apparatus) functions to expose a target region by gradually scanning the reticle pattern in any one scanning direction and driving a wafer stage in the same or opposite direction.
Modern semiconductor devices are formed with a plurality of pattern layers, with each layer formed using similar procedures as described above. As feature sizes of the circuits formed on the wafers shrink, however, proper alignment of the multiple patterns becomes more difficult. Current techniques form a single microscopic alignment mark on the reticle and use that to track proper alignment during multiple exposures in the same target region.
Alignment using the scanner-type exposure apparatus has been found to be even more difficult since the high-powered KrF or ArF excimer laser light sources used during lithography heat the lens used to focus the light. Such heated lenses exhibit changed localized indices of refraction and other unpredictable optical aberrations. Such aberrations pose very serious problems, affecting overlay alignment when manufacturing a nano-scaled semiconductor device requiring an overlay margin of no more than 15 nm.
Overlay errors due to aberration generated at this time are classified into field translation, field rotation, field magnification, and third order distortion, as shown in FIGS. 1A, 1B, 1C, and 1D, respectively. While the field translation, the field rotation, and the field magnification can be relatively easily controlled by the conventional optical systems, the third order distortion, which is a mixed form of pincushion distortion and barrel distortion, cannot be easily controlled.
The third order distortion may be somewhat controlled by measuring X and Y coordinates between the reticle and the wafer stage (or the wafer). Current methods form a plurality of alignment marks at the reticle, install a detection sensor at the wafer stage to measure coordinates, and perform alignment thereby.
One drawback to this method, however, is that third order distortions increase the alignment error differential as the X and Y coordinates become even more asymmetrical with respect to a center of measured positions. This increasing alignment error caused by third order distortion can be seen from the following Formulae 1 in which error values corresponding to X and Y-axes are given by third order polynomial functions.X-axis error=AX3+BX2+C, Y-axis error=A′Y3+B′Y2+C′  (1)
The above formulae show that when two coaxial alignment marks, located opposite to each other, are located at different positions about a center of measured positions, the larger the third order distortion constant A or A′, the larger the difference between alignment errors.
Describing an example of alignment errors, the X-axis alignment mark having a plurality of segments formed in the X-axis direction and the Y-axis alignment mark having a plurality of segments formed in the Y-axis direction are horizontally aligned to be formed one by one in the same shape in all quadrants of the reticle, thereby allowing their alignment state to be measured. That is, in all quadrants, the Y-axis alignment mark is formed at a left side, and the X-axis alignment mark is formed at a right side, of the Y-axis alignment mark.
Here, the alignment state in the Y-axis direction has no alignment error since each segment is located at the same distance from the Y-coordinate axis of the reticle. However, the X-axis alignment mark has an alignment error since the segments are located at different distances from the X-coordinate axis. Moreover, the larger constant values of third order distortion become (A1<A2<A3), as shown in Formulae 1, the larger alignment error becomes as shown in FIGS. 2A and 2B.
In other words, when an X coordinate is located at a distance of 8.9 mm to the right of an optical axis on a wafer surface and another X coordinate is located at a distance of 12.9 mm to the left of the optical axis, error of the left alignment mark is about 0.2 μm in the case of the constant A1 of the third order distortion, about 0.38 μm in the case of the constant A2, and about 0.6 μm in the case of the constant A3. Meanwhile, error of the right alignment mark is about 0.08 μm in the case of A1, about 0.13 μm in the case of A2, and about 0.24 μm in the case of A3. In other words, the errors are not symmetric about the X-axis.
Finally, as shown in FIG. 2B, the alignment errors about a center (or the optical axis) of the reticle become gradually larger with an increasing distance from the X-axis. Therefore, in order to solve the problems of the conventional art, the present invention provides a reticle and an exposure apparatus using the reticle.
Accordingly, the need remains for methods that assist in overcoming errors, especially asymmetric errors, in alignment caused by third order distortions.